Throughout this application, various publications, patents and published patent applications are referred to by an identifying citation. The disclosures of the publications, patents and published patent applications referenced in this application are hereby incorporated by reference into the present disclosure.
Conductivity measurements of a chemical solution may be made by applying a voltage across a pair of electrodes and immersing them in the solution. The electric current passing through the system is proportional to the conductivity of the solution. This technique, however, is not optimal if the solution to be measured is chemically incompatible with the metallic electrodes, e.g., resulting in chemical attack or contamination of the solution and/or electrodes.
Another approach involves an electrodeless toroidal conductivity measurement. In this approach, an electric transformer is effectively created through the use of driver and sensor toroidal coils surrounding a ‘core’ formed at least partially by the solution under test. The toroids are typically disposed within an electrically insulative, magnetically transparent housing having a fluid flow path which passes axially therethrough. The driver is supplied with a voltage which induces an electromagnetic field in the solution passing through the flow path, which then induces a current in the sense coil. The induced current is proportional to the conductivity of the solution being measured.
An example of such a toroidal conductivity sensor is disclosed in Reese, U.S. Pat. No. 5,157,332. A commercial example of a similar sensor is known as the 871EC™ invasive conductivity sensor available from Invensys Systems, Inc. (Foxboro, Mass.). As shown in FIG. 1, a section of such an electrodeless conductivity sensor 20 includes toroidal coils 11, 12, 13 encased in a housing 21, which may be immersed in the fluid to be measured. The housing 21 defines a central bore 19 which allows fluid to pass axially through the toroids 11, 12, 13, without contacting them. The induction loop of the ‘core’ is completed by the process solution within which the sensor is immersed.
Where a fluid to be measured is flowing through a conduit, it may not be possible or desirable to immerse a sensor in the fluid. In this event, driver and sensor toroidal coils may surround a pipe carrying the liquid. A commercial example of such a sensor is known as the 871FT™ (Invensys Systems, Inc.). However, in order for induction to occur, an electrical loop must be completed outside the coils, typically by clamping a metallic strap to metallic portions of the pipe upstream and downstream of the toroids. A drawback of this approach, however, is that metallic pipe portions cannot be used when the process fluid attacks or is otherwise incompatible with metals.
Alternatively, the induction loop may be completed by the fluid itself, by providing a secondary flow path that bypasses one or more of the toroids. An example of such a fluid loop is disclosed in U.S. Pat. No. 2,709,785 to Fielden. A drawback of this approach is that the limited cross section, relatively long length and high resistance of the fluid itself, adds a net resistance to the induced current which tends to adversely affect the sensitivity of conductivity measurement. Approaches intended to enhance the sensitivity of conductivity sensors include that disclosed by Ogawa, in U.S. Pat. No. 4,740,755. Ogawa discloses toroids on a fluid loop with dimensions calculated to “provide a low value for the ratio of the length of fluid flow loop . . . to the cross sectional area of the flow path, which in turn provides good sensitivity.” (Ogawa col. 2 lines 42-47). A drawback to this approach is that Ogawa's toroids are taught to be coplanar and physically separated in order to reduce leakage coupling between the transformers. (Ogawa at col. 1, lines 34-38, col. 2 lines 47-52, col. 4, lines 49-55).
The aforementioned corrosion and contamination issues also make temperature detection problematic for applications involving a metallically incompatible process fluids. This incompatibility militates against the conventional placement of metallic temperature detectors within the fluid flow path. One approach intended to avoid this problem is to place the temperature detector on the outside wall of the fluid flow conduit and to thus rely on thermal conduction through the conduit wall to obtain temperature data. However, conduits containing corrosive fluids are often fabricated from polymers such as PFA (perfluoroalkoxy polymer resin), PTFE (polytetrafluoroethylene), polyvinyl chloride (PVC), or various combinations thereof, such as perfluoroalkoxy-polytetrafluoroethylene co-polymer. The relatively poor thermal conductivity of these materials tends to adversely affect the accuracy and response time provided by such external temperature detection approaches.
A need therefore exists for a system of measuring the temperature of a process fluid flowing through a conduit, which addresses one or more of the abovementioned drawbacks.